A probe head of the afore-mentioned kind is disclosed in EP 0 483 228 B1 (U.S. Pat. No. 5,298,864) and in DE 44 42 742 C1 (U.S. Pat. No. 5,760,586).
In the field of nuclear magnetic resonance measurements (NMR) it is well known to let the sample under investigation rotate, and to arrange same in an orientation relative to the constant magnetic field (B0) which, instead of the conventional 90°, is inclined under the so-called “magic angle” of 54.7°. This type of measurement method is conventionally identified as “magic angle spinning” (MAS).
In prior art MAS setups of the type specified at the outset, for exciting a nuclear resonance, a high frequency magnetic field (B1) of a frequency in the range of at most 300 to 400 MHz is used, which, for protons (1H) corresponds to a constant magnetic field (B0) of between 7 and 9.4 T. For that purpose, solenoid coils are used, being arranged within the stator and coaxially surrounding the rotor. The solenoid coil generates the high frequency magnetic field (B1) required for the NMR measurement, and, simultaneously, serves as a receiving coil for the nuclear resonance signal of even frequency generated by the sample substance.
Due to progress in the field of magnet design, however, it has become conventional in the meantime to generate constant magnetic fields (B0) with a field strength in the area of up to 22 T, which, for protons (1H) corresponds to a resonance frequency of about 940 MHz.
In this frequency range, solenoid coils are difficult to handle, because their dimensions become too small, so that no sufficient high frequency field strength (B1) may be generated, and, further, they may hardly be tuned. Moreover, for such high frequencies, the areas of the high frequency electrical and the high frequency magnetic fields more and more overlap in solenoid coils, such that an increasing portion of the sample substance is located in the area of the high frequency electrical field. This, in turn, results in high dielectric losses, not only for solid samples but in particular for liquid and for electrically conductive, for example salt-containing samples, all of which results in a deterioration of the quality factor of the resonance structure, which, in turn, worsens the measuring sensitivity. In probe heads with MAS function, these circumstances present problems already in the frequency range above 750 MHz (17.6 T).
In the field of electron spin resonance spectroscopy (ESR), various resonance structures are known for the microwave frequencies used therein, in particular within the X-band (about 9 to 12 GHz). Besides the conventionally used hollow cavity resonators, also so-called “split-ring” resonators are used in the field of ESR, as, for example, described in U.S. Pat. Nos. 4,446,429 and 5,744,957. Apart from that, it is also known in the art of ESR to use dielectric resonators, made from sapphire, such as described in an article described by Biehl, R. “Sensitivity enhancement in EPR—The dielectric ring TE 011 cavity”, Bruker Report, 1/1986, pp. 45-47, as well as in DE 30 29 754 C2 (U.S. Pat. No. 4,437,063), and in DE 41 25 655 C2 (U.S. Pat. No. 5,345,203). With all these prior art arrangements, the particularly described resonance structure exclusively has the function to configure an electrically substitute for conventional hollow cavity resonators. Further functions, in particular mechanical auxiliary functions, are not provided.
U.S. 2003/0052678 A1 discloses an NMR probe head for being used within an ultracentrifuge. The set up comprises a hollow cylindrical stator consisting of an electrically conductive, however non-magnetic material. A rotor is contained within the stator, the rotor being made from an electrically non-conductive and non-magnetic material. An electrical conductor extends along the central axis of the cylindrical stator. The sample is positioned within the rotor. The stator and the electrical conductor, together, generate the high frequency magnetic field as required for exciting the nuclear resonance. The rotor containing the sample rotates within the stator and is journalled therein by means of an air bearing generated between the stator and the rotor.